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ATEL # 2547; D. M. Russell (University of Amsterdam), M. Buxton (Yale University), F. Lewis (Faulkes Telescope Project, University of Glamorgan, Open University), D. Altamirano (University of Amsterdam)
on 12 Apr 2010; 16:49 UT
Password Certification: David M. Russell (D.M.Russell@uva.nl)
Subjects: Infra-Red, Optical, Black Holes, Transients
Referred to by ATEL #: 2573
The transient black hole candidate GX 339-4 is currently softening in X-ray, making a transition out of the hard state (ATel #2545). We are monitoring the source with the Faulkes Telescope South (located at Siding Spring, Australia) and the SMARTS 1.3m telescope and ANDICAM instrument (at Cerro Tololo, Chile). From the Faulkes monitoring, the source brightened in the hard state over the last five months from a faint (V = 19.6, R = 18.8, i' = 18.6; errors are 0.1 mag at most) magnitude on 2009-10-22 = MJD 55126 (ATel #2270) to a bright (V = 15.1, R = 14.5, i' = 14.3, V-i' = 0.79) one on 2010-04-01 = MJD 55287 (with a very bright radio jet; ATel #2525). One week later on 2010-04-08 = MJD 55294 the source had faded and the colours were slightly bluer than before (V = 15.3, R = 14.7, i' = 14.6, V-i' = 0.65). Meanwhile, the X-ray flux (RXTE ASM, PCA Bulge scan, Swift BAT, MAXI) continued to increase or remain at about the same level during this time.
Finally, the source has dramatically faded by one magnitude in the last three days. From Faulkes we measure R = 15.7 +- 0.1, i' = 15.7 +- 0.2 on 2010-04-11 = MJD 55297.8 and from SMARTS, V = 16.38 +- 0.05, H = 13.16 +- 0.10 on 2010-04-12 = MJD 55298.3. A link to our Faulkes telescope light curves is provided below.
The rapid drop in optical flux and colour change at the start of the state transition is reminiscent of previous outbursts (Coriat et al. 2009) and is an indication that the OIR-emitting synchrotron jet is fading. The optical and infrared flux will probably continue to drop in the coming days. The jet is likely to persist at radio frequencies for the next week or two before the bright, optically thin radio flare(s), which we predict will occur around ~ 17 - 22 April (Russell & Fender 2007). We note that the softening of the X-ray flux at this transition is occurring at a slightly lower flux than the 2002-3 and 2007 outbursts but at a brighter flux than the 2004-5 outburst (see a compilation of X-ray light curves and hardness-intensity diagrams here). Regular radio monitoring is strongly encouraged during the next month.
In addition, the optical counterpart is variable on short timescales. From 31 Faulkes i'-band images on 2010-04-08 (each of exposure time 30 sec; the time resolution was 90 sec) we measure an rms variability amplitude of 0.09 mag (a fainter field star had an rms variability amplitude of 0.01 mag, similar to the relative errors of the magnitude measurements).
The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCO GTN). DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.
Faulkes optical light curves